I wonder if the 4:3 versions on the FX app are the original broadcast versions without the aforementioned changes… Those are the ones I watch and they seem to be original broadcast versions as many of those streaming episodes still have the “Presented in Dolby Surround” logo at the start of the episode. The audio definitely has a different sound than that of its 16:9 counterpart on the app.
From what Ive heard the FXX 4:3 versions come from the DVD masters but I don’t think anyone has done a full check. I know at the very least, hitlers birthday/lassies birthday is the same on the DVD as FXX.